How do you animate image texture materials in blender cycles with keyframes?
To dissolve one image into another, connect the different image textures to a color mix node, before the shader node.
By animating the Mix Factor you can control the how the images change.



Cascading this node setup might work for a few images.



But if you have many images, you might want to turn them into an image sequence (just renaming them with the same name and progressive numbers will work). Then load the image sequence as a texture.

If what you want to do is have the image change with every frame of your scene use the default options and use autorefresh.



If you want to manually set the duration of each image. Set the number of frames and start frame to 1 and animate the offset value.



Note that by doing this the images will change from one frame to the next but will not dissolve gradually.